Nickel-chromium alloys useful in the production of wrought articles for high temperature application



United States The present invention relates to age-hardenable alloysparticuarly suitable for use at elevated temperatures and, moreparticularly, to age-hardenable, nickel-base alloys having anadvantageous combination of mechanical characteristics at elevatedtemperatures.

Heretofore, the art has endeavored to provide alloys suitable for hightemperature applications and particularly alloys to be used as materialsfor rotor discs in gas turbines. The special requirements imposed onalloys for this purpose have been discussed in US. Patent No. 3,151,981of R. A. Smith and I. Heslop. The alloys described and claimed thereincontain from 0.02 to 0.09% carbon, from 14 to 22% chromium, from 10 to20% cobalt, from 3 to 10% molybdenum, from 2 to 3.5% titanium, from to0.8% aluminum, the sum of the titanium and aluminum contents beinggreater than 2.5%, from 2 to 5.25% columbium, from 0 to 25% iron, from0.001 to 0.01% boron, and from 0.01 to 0.1% zirconium, the balance,apart from impurities and residual deoxidants, being nickel. Molybdenummay be replaced by an equal atomic percentage of tungsten up to amaximum tungsten content of by weight, and columbium by an equal Weightof tantalum up to a maximum tantalum content of 3%. In order to avoidthe formation of embrittling phases in the above-mentioned alloys, thecontents of iron and columbium in these alloys have to be speciallycorrelated within the ranges set forth, the highest columbium contentsbeing associated with the lowest iron contents and vice versa.Increasing the iron content, while making the alloys cheaper, alsoreduces their tensile strength and proof stress at high temperatures,and for the highest strength the iron content should not exceed 5%.

While the tensile and creep properties of alloys of the type set forthin the aforementioned patent are excellent up to temperatures of about600 0, these properties fall off at higher temperatures, and thecontinued development of gas turbine engines has given rise to a needfor alloys having greater strength at temperatures above 600 C. Althoughsome improvement in tensile strength results from increasing thetotalcontent of the hardening elements titanium and aluminum, this has beenfound to lead to a drastic reduction in tensile ductility, and to makethe alloys almost unforgeable. Although many attempts were made toovercome the foregoing difliculties and other disadvantages, none, asfar as we are aware, was entirely successful when carried into practicecommercially on an industrial scale.

It has now been discovered that age-hardenable, nickelbase alloys,having excellent mechanical properties at temperatures above 600 C., canbe produced by carefully controlling and restricting the ranges ofspecific alloying '7 atent C 3,343,950 Patented Sept. 26, 1967 iceelements contained therein, such as chromium, titanium, aluminum,columbium and vanadium. By way of further explanation, it has been foundthat if, at the same time as the total content of aluminum and titaniumis increased, the ratio of aluminum to titanium is increased and thecontent of columbium is decreased, the tensile strength of the alloys athigh temperatures is improved while adequate ductility is stillretained. Titanium can indeed be entirely absent. This discovery isdoubly surprising, since in the alloys of our previous specification, inwhich titanium predominated, on the one hand aluminum contents above0.8% greatly reduced the tensile ductility, and on the other hand toolow a columbium content was associated with a low tensile strength andproof stress. When the composition of the alloys is carefully controlledin this way the chromium content must also be controlled in order toavoid embrittlement of the alloys on prolonged exposure to hightemperatures. Furthermore, to ensure that the alloys are notnotch-sensitive, that is to say that their stress-rupture life is notreduced by the presence of notches, the total content of the hardenerelements titanium, aluminum, columbium and vanadium must be restricted.

It is an object of the present invention to provide an age-hardenable,nickel-base alloy having an advantageous combination of mechanicalcharacteristics at temperatures above about 600 C.

Another object of the invention is to provide an agehardenable,nickel-base alloy having an advantageous combination of mechanicalcharacteristics at temperatures above about 600 C. and which isparticularly resistant to high temperature embrittlement and isnotch-insensitive.

The invention also contemplates providing a rotor disc formed from anage-hardenable, nickel-base alloy having an advantageous combination ofmechanical characteristics at temperatures above about 600 C. and whichis particularly resistant to high temperature embrittlement and isnotch-insensitive.

Other objects and advantages Will become apparent from the followingdescription.

Generally speaking and in accordance with the present invention, alloyscontemplated herein advantageously contain, in percentages by weight,from about 11% to about 16% cobalt, from about 10.1% to about 17%chromium, from about 0.02% to about 0.1% carbon, from about 5% to about9% molybdenum, from about 2.53% to about 5.2% aluminum, up to about1.73% titanium, about 1% to about 2.5% columbium, up to about 1.75%vanadium, about 0.001% to about 0.01% boron, about 0.01% to about 0.1%Zirconium, the balance being essentially nickel. The term balanceessentially nickel,'as commonly understood by those skilled in the art,does not exclude small amounts of other incidental elements, e.g.,deoxidizers and impurities, which are commonly present in nickel alloysand do not adversely affect the novel charateristics of the presentinvention. In this regard, small amounts of manganese and silicon, e.g.,up to about 1% of each, are tolerable in the present alloys. Iron is anundesirable impurity in the present alloys since it increases theirtendency to embrittle on prolonged exposure to high temperatures, andits content should therefore not exceed 1%, though greater amounts, upto 5%, may be tolerated J at chromium contents within the lower part ofthe range. Preferably, the total content of all impurities and residualdeoxidants does not exceed 2%.

Within the limits set out above, the amounts of aluminum and titaniummust be such that the ratio of aluminum to titanium (Al:T i) is at least2:1 by weight and the total content of aluminum and titanium (AH-Ti) isfrom 3.8% to 5.2%. At lower ratios of aluminum to titanium the hightemperature tensile strength of the alloys falls off very sharply. Atotal aluminum-l-titanium content of at least 3.8% is needed to ensurethat the alloys have adequate strength at high temperatures, but if thistotal exceeds 5.2% the tensile ductility of the alloys at hightemperature is reduced and hot working becomes very difiicult or evenimpossible. Columbium and vanadium also contribute to the strength ofthe alloys, and to obtain adequate high temperature strength theseelements must be present in a total amount (Cb-t-V) of at least 2%.

The substitution of vanadium for an equal weight of columbium in thealloys improves their tensile ductility and hot workability, andpreferably the alloys contain at least 0.5% vanadium. On the other hand,vanadium impairs the high temperature tensile and creep strengths of thealloys and reduces their resistance to oxidation. For this reason thevanadium content must not exceed 2% and the ratio of vanadium tocolumbium (VzCb) must not exceed 1.5:1.

Commercially available sources of columbium are usually contaminated bytantalum and small amounts of tantalum will, therefore, usually bepresent in the alloys of the invention. If desired, tantalum may also bedeliberately added to the alloys so that up to one half or even thewhole of the columbium content is replaced atom for atom by tantalum.

Small additions of boron and zirconium have markedly beneficial etfectson tensile ductility at temperatures of 600 C. and above, and both ofthese elements must be present. However, as the content is increased,the melting point of the alloys falls, and if more than 0.1% boron or0.1% Zirconium is present, serious deterioration in the hot workingproperties of the alloys results.

In addition to the need to restrict the contents of the main hardeningelements in the manner just described, it is also important that thecontents of each of the other constituents of the alloy should be withinthe limits set forth.

At chromium contents less than about of the resistance of the alloys tooxidation, and particularly to attack by the products of combustion ofgas turbine fuels, decreases, and preferably the chromium content is atleast 12%. If the chromium content exceeds 17% the alloys become brittleon prolonged exposure to high temperatures, and preferably the chromiumcontent does not exceed 16%.

The cobalt content has a very marked elfect on the strength of thealloys. Their high temperature tensile strength is reduced when thecobalt content is decreased below 11% or increased above 16%, andpreferably the cobalt content is from 13% to 15 Molybdenum has abeneficial effect on both tensile strength and ductility at hightemperatures, and the presence of at least 5% molybdenum is verydesirable. On the other hand, excessive additions of molybdenum carrythe penalties of increased density, decreased machinability andincreased ditficulty in hot working, and the content should, therefore,not exceed 9%.

In the alloys of this invention tungsten is not equivalent to molybdenumand its presence has many disadvantages. It reduces the tensileductility of the alloys, increases their density and thus the stress inrotating components made from them, and impairs the workability of thealloys. Nevertheless small amounts of tungsten, up to a maximum of 2%,can be tolerated as a replacement for half its weight of molybdenum.Preferably, however, the alloys are entirely free from tungsten.

In addition, the total amount of the hardening elements aluminum,titanium, columbium and vanadium is also important. To ensure adequatehigh temperature strength this total must be at least 6.3%. Increasingthe total hardener content increases the high temperature strength, butat the same time it tends to make alloys notchsensitive, reduces theirtensile ductility at high temperatures, increases their tendency toembrittle, and makes them harder to forge. For these reasons the totalhardener content must not exceed 8.5% or even 8%, and prefera'bly it isnot more than 7.5%

In carrying the invention into practice, the alloys can be air melted,but to ensure the best creep properties and workability they arepreferably melted and cast under vacuum. If they are melted in air theyare preferably deoxidized by means of calcium or magnesium and refinedby holding under vacuum in the molten state for some time beforecasting. The pressure during this refining treatment should not be morethan 0.1 mm. Hg and is preferably lower, e.g. 5 microns or less; thetemperature is suitably from 1400 C. to 1600 C.; and the holding time isat least 5 minutes and preferably is at least 10 minutes. The castingots may be processed to bar or rotor disc form by conventionalextrusion, forging or pressing techniques and may be further processedto sheet by extrusion, forging, hot rolling and col-d rolling.

The alloys of the present invention are of the age hardenable type andrequire suitable heat treatment in order to develop the criticalcombination of properties required. Both solution heating and agingtreatments are required. The former is most important since it largelydecides the values of creep strength and proof strength that can beachieved in a given alloy. Very high solution heating temperatures givethe highest possible creep re-. sistance, while on the other hand lowersolution heating temperatures favor increased proof strength. A suitableheat treatment for parts made from the alloys comprises solutiontreatment for from V2 to 8 hours at 900 C. to 1150 C., followed bycooling at any practical rate (e.g., by air cooling or oil quenching or,in the case of sheet, water quenching) and then aging. Aging treatmentscan be carried out at temperatures in the range of 650 C. to 850 C. or900 C. and may involve one or more stages of heating for from 4 to 40hours at successively lower temperatures in this range. A preferred heattreatment to give the highest level of proof strength, together withreasonable creep strength, consists in solution heating for from 1 to 8hours at 1050 C. to 1150 followed by aging for from 4 to 20 hours at 750C. to 850 C.

For the purpose of giving those skilled in the art a betterunderstanding of the invention and/ or a better appreciation of theadvantages of the invention, the fol lowing illustrative examples aregiven:

Example I A series of tensile tests was carried out at 750 C. and impacttests at room temperature on a number of alloys that contained, besidesthe elements set forth in Table I, 0.05% carbon, 14% cobalt, 7%molybdenum, 0.003% boron, 0.05 zirconium, balance nickel. The testpieces were machined from forged bar of the alloys, and the tensile testpieces were given the following heat treatment before testing; solutionheated for 1 hour at 1130 C., air cooled, aged for 4 hours at 820 C.,air cooled, again aged for 16 hours at 700 C. and air cooled.Elongations were measured on a gauge length of one inch.

The impact tests were performed on Charpy V-notch test pieces that hadbeen solution-heated for 1 hour at 1130 C., air cooled, and then heatedat 750 C. for 1000 hours to simulate long-time exposure in service. Theresults of these tests are set forth in Table 1:

No. 10, with 20% chromium, contained sigma phase at all four of thetemperatures used.

TABLE I Tensile Properties at 750 0. Alloy Impact No. Cr Ti Al Nb VStrength Y.S. U.T.S. Elong. (ft-lb.) (tons/in?) (tons/in!) (percent) 0.5 4.0 1. 5 1. 5 64 74 5 10 0. 5 4.0 l. 5 l. 5 61 74 12 6. 5 0. 5 3. 5 l.5 l. 5 58 63 ll 12 0.5 4.0 1.0 1.0 58 70 10 9.4 0.5 4.0 2. 2.0 66 76 73. 6 0.5 4.0 1. 1. 5 64 70 2.1 3. 5 1.0 l. 5 1. 5 51 66 3 5. 8 2. 5 2.01.5 l. 5 56 67 10 2. 5 0. 5 4. 75 49.6 64 7 1 Molybdenum content 5 0.

Tensile Strength '1.S.I.=Long tons (2,240 lbs.) per square inch.

Alloys Nos. 1 to 5 are in accordance with the present invention, whileNos. 6 to 9 are not. Alloys 7 and 8 (as does Alloy 9) deviate from theinvention in having an aluminum to titanium ratio (Al:Ti) of less than2:1, while Alloy 6 has too high a chromium content. It is readilyapparent that alloys within the invention exhibit high temperaturemechanical properties and resistance to embrittlement superior to thoseoutside the invention, which include No. 9, one of the best disc alloyshitherto available and which is in accordance with our US. Patent No.3,151,981. This is true despite the fact that Alloy No. 9 was melted andcast under vacuum, whereas all the others in the above table were meltedand cast 1n air.

Comparison of the results for Alloy No. 2 with those 5 of No. 7 showsthe much poorer tensile properties that result from the use of AlzTiratios less than 2: 1.

Alloy No. 6, with 18% chromium, has very poor impact properties, andcomparison of the impact properties of Alloys Nos. 4, 2 and 5 show howthe impact strength decreases as the total content of niobium andvanadium is increased. Comparison of Alloy No. 2 with No. 3 shows thatdecrease in the aluminum content somewhat improves the impact strengthat the expense of some loss in strength. The best all-round combinationof properties is shown by Alloy No. 1.

Example II A series of Charpy V-notch tests were carried out to furtherillustrate the particularly good resistance to notch sensitivity and tohigh temperature embrittlement of an alloy containing chromium ascompared with alloys having higher chromium contents. The alloys testedcontained, besides the amount of chromium set forth in Table II, 0.05%carbon, 14% cobalt, 7% molybdenum, 0.5% titanium, 4.0% aluminum, 1.5%columbium, 1.5% vanadium, 0.003% boron, 0.05% zirconium, balance nickel.Each alloy was tested at room temperature after heating for a prolongedperiod of 1000 hours at one or X-ray examination of the specimens ofAlloy No. 2 (15% chromium) heated at 750 C. and 900 C. did not revealany of the embrittling sigma phase, but the Alloy Example IIIStress-rupture tests were carried out at a stress of 33.5 t.s.i. and atemperature of 732 C. on un-notched and notched samples of Alloy No. 2,after a heat treatment consisting of solution heating at 1130 C. for 1hour, aircooling, aging at 850 C. for 5 hours and again air-coolingshowed that this alloy is notch-strengthened. The unnotched specimenbroke after 308 hours with an elongation of 4.3%, while the notchedspecimen was unbroken after 600 hours.

A most advantageous range of alloy composition contemplated herein foralloys having an advantageous combination of properties, including hightemperature strength and ductility combined with resistance to notchsensitivity and high temperature embrittlement, is as follows: about0.04% to about 0.06% carbon, about 12% to about 16% chromium, about 13%to about 15% cobalt, about 6% to about 8% molybdenum, up to about 0.6%titanium, about 3.7% to about 4.8% aluminum, the total content ofaluminum and titanium (Al+Ti) being about 4% to about 5%, about 1.4% toabout 1.7% columbium, about 1.2% to about 1.7% vanadium, the totalcontent of aluminum, titanium, columbium and vanadian being about 6.6%to about 7.4%, about 0.001% to about 0.01% boron, about 0.01% to about0.1% zirconium, balance essentially nickel. A particularly advantageouscomposition consists essentially of about 0.05% carbon, about 15chromium, about 14% cobalt, about 7% molybdenum, about 0.5% titanium,about 4.0% aluminum, about 1.5% columbium, about 1.5% vanadium, about0.003% boron, about 0.05% zirconium, balance essentially nickel.

While the alloys of the invention are particularly suitable for makingwrought rotor discs for gas-turbine engines, their advantageouscombination of properties also makes them useful as material for largebolts for service at elevated temperatures, for example, for securingthe casings of steam turbines. The threads of such bolts constitutenotches, so it is most important that the alloy used be notnotch-weakened.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. An age-hardenable alloy having an advantageous combination ofmechanical characteristics at temperatures above about 600 C. and whichis further characterized by notch insensitivity and by resistance tohigh temperature embrittlement, said alloy consisting essentially ofabout 0.02% to about 0.1% carbon, about 10% to about 17% chromium, about11% to about 16% cobalt, about to about 9% molybdenum, about 2.53% toabout 5.2% aluminum, up to about 1.73% titanium, the total content ofaluminum and titanium being about 3.8% to about 5.2% and the ratio ofaluminum to titanium being at least 2:1, about 1% to about 2.5%columbium, up to about 2% vanadium, the total content of columbium andvanadium being about 2% to about 4% and the ratio of vanadium tocolumbium not exceeding 1.5 :1 about 0.001% to 0.01% boron, about 0.01%to about 0.1% zirconium, and the balance essentially nickel, the totalcontent of aluminum, titanium, columbium and vanadium being about 6.3 toabout 8.5%.

2. An alloy as set forth in claim 1 in which the vanadium content is atleast 0.5%.

3. An alloy as set forth in claim 1 wherein columbium is at leastpartially replaced by an equal atomic percentage of tantalum.

4. An alloy as set forth in claim 1 wherein molybdenum is partiallyreplaced by an equal atomic percentage of tungsten up to a maximum. of2% by weight of tungsten.

5. As a new article of manufacture, a turbine rotor disc formed from thealloy set forth in claim 1.

6. An age-harden-able alloy having an advantageous combination ofmechanical characteristics at temperatures above about 600 C. and whichis further characterized by notch insensitivity and by resistance tohigh temperature embrittlement, said alloy consisting essentially ofabout 0.04% to about 0.06% carbon, about 12% to about 16% chromium,about 13% to about 15% cobalt, about 6% to about 8% molybdenum, about3.7% to about 4.8% aluminum, up to about 0.6% titanium, the totalcontent of aluminum and titanium being about 4% to about 5%, about 1.4%to about 1.7% columbium, about 1.2% to about 1.7% vanadium, about 0.001%to about 0.01% boron, about 0.01% to about 0.1% zirconium, and thebalance essentially nickel, the total content of aluminum, titanium,columbium and vanadium being about 6.6% to about 7.4%.

7. As a new article of manufacture, a turbine rotor disc formed of thealloy set forth in claim 6.

8. An age-hardenable alloy having an advantageous combination ofmechanical characteristics at temperatures above about 600 C. and whichis further characterized by notch insensitivity and resistance to hightemperature embrittlement, said alloy consisting essentially of about0.05% carbon, about 15% chromium, about 14% cobalt, about 7% molybdenum,about 4% aluminum, about 0.5% titanium, about 1.5% columbium, about 1.5%vanadium, about 0.003% boron, about 0.05% zirconium, and the balanceessentially nickel.

References Cited UNITED STATES PATENTS 3,061,426 10/1962 Bie'ber -1713,151,981 10/1964 Smith @1211 75-171 3,155,501 11/1964 Kaufman et a1.75171 3,166,412 1/1965 Bieber 75 17 1 HYLAND BIZOT, Primary Examiner.

R. O. DEAN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No,5,545,950 September 26, 1967 Edward Gordon Richards et a1 It is herebycertified that error appears in the above numbered patent requiringcorrection and that the said Letters Patent should read as correctedbelow.

in the heading to the printed specification, lines 6 and 7 for"International Nickel Company" read The International Nickel Company,Inc column 2, line 45, for "10.1%" read 10% lines 56 and S7, for"charateristlcs" read -character1st1cs column 5, line 59, for "0H 1%boron" read 0.01% boron line 47, for "10% of the" read 10% the column 7,line 11, after "1.5:1" insert a comma.

Signed and sealed this 8th day of October 1968 (SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD BRENNER Attesting Officer Commissioner ofPatents

1. AN AGE-HARDENABLE ALLOY HAVING AN ADVANTAGEOUS COMBINATION OFMECHANICAL CHARACTERISTICS AT TEMPERATURES ABOVE ABOUT 600*C. AND WHICHIS FURTHER CHARACTERIZED BY NOTCH INSENSITIVITY AND BY RESISTANCE TOHIGH TEMPERATURE EMBRITTLEMENT, SAID ALLOY CONSISTING ESSENTIALLY OFABOUT 0.02% TO ABOUT 0.1% CARBON, ABOUT 10% TO ABOUT 17% CHROMIUM, ABOUT11% TO ABOUT 16% COBALT, ABOUT 5% TO ABOUT 9% MOLYBDENU, ABOUT 2.53% TOABOUT 5.2% ALUMINUM, UP TO ABOUT 1.73% TITANIUM, THE TOTAL CONTENT OFALUMINUM AND TITANIUM BEING ABOUT 3.8% TO ABOUT 5.2% AND THE RATIO OFALUMINUM TO TITANIUM BEING AT LEAST 2:1, ABOUT 1% TO ABOUT 2.5%COLUMBIUM, UP TO ABOUT 2% VANADIUM, THE TOTAL CONTENT OFCOLUMBIUM ANDVANADIUM BEING ABOUT 2% TO ABOUT 4% AND THE RATIO OF VANADIUM TOCOLUMBIUM NOT EXCEEDING 1.5:1 ABOUT 0.001% TO 0.01% BORON, ABOUT 0.01%TO ABOUT 0.1% ZIRCONIUM, AND THE BALANCE ESSENTIALLY NICKEL, THE TOTALCONTENT OF ALUMINUM, TITANIUM, COLUMBIUM AND VANADIUM BEING ABOUT 6.3%TO ABOUT 8.5%.